Proper cardiac function relies on the synchronized contractions of the heart at regular intervals. When the heart is functioning normally, synchronized cardiac contractions are initiated at the sinoatrial node and the heart is said to be operating in normal sinus rhythm. However, if contractions of the heart become irregular or uncoordinated, or if the contraction rate is too fast or too slow, the heart rhythm is described as arrhythmic. Cardiac arrhythmia may be caused, for example, by disease processes or from aberrant electrical conduction patterns occurring in the heart tissue. Cardiac arrhythmia impairs cardiac pumping efficiency and some types of cardiac arrhythmia can be life threatening.
A cardiac arrhythmia that originates in an atrial region of the heart is denoted a supra-ventricular tachyarrhythmia (SVT). Atrial fibrillation and atrial flutter are examples of SVT. Both conditions are characterized by rapid, uncoordinated contractions of the atria resulting in hemodynamically inefficient pumping action.
Another example of SVT is sinus tachycardia, which is an increased heart rate due to exercise or a quick emotional response. In contrast to atrial fibrillation and atrial flutter, sinus tachycardia is characterized by rapid, coordinated contractions of the atria resulting in hemodynamically efficient pumping action, compensating for the increased strain placed upon the body during exercise or quick emotional responses. Whereas atrial fibrillation and atrial flutter are “abnormal” (yet not lethal), sinus tachycardia is “normal” (and also not lethal).
Cardiac arrhythmias originating in a ventricular region of the heart are denoted ventricular tachyarrhythmias. Ventricular tachycardia (VT) is characterized by rapid ventricular contractions and can degenerate into ventricular fibrillation (VF). Ventricular fibrillation produces extremely rapid, non-coordinated contractions of the ventricles. Ventricular fibrillation is fatal unless the heart is returned to sinus rhythm within a few minutes.
Implantable medical devices, including pacemakers and implantable cardioverter/defibrillators (ICDs), and have been used to deliver effective treatment to patients with serious cardiac arrhythmias. Implantable medical devices may treat cardiac arrhythmias with a variety of tiered therapies. These tiered therapies range from delivering low energy pacing pulses timed to assist the heart in maintaining pumping efficiency to providing high-energy shocks to treat and/or terminate fibrillation. To effectively deliver these treatments, the ICD must first identify the type of arrhythmia that is occurring, after which appropriate therapy may be delivered to the heart.
In addition to arrhythmic episodes, patients may experience other types of cardiac episodes. Various types of cardiac episodes may be detected by analysis of various sensor signals available to the ICD, such as cardiac electrical signals sensed by the ICD electrodes, hemodynamic sensor signals, activity signals, posture signals, respiration signals, and/or signals produced by other types of sensors. For example, patients suffering from congestive heart failure may experience periods of cardiac decompensation when the heart fails to adequately pump blood to the tissues of the body. In another example, patients may experience myocardial ischemia when there is insufficient blood supply to the heart. If the blood supply is severely interrupted, myocardial infarction may occur. These episodes are detectable by characteristic sensor signals present before, during, and/or after the episode occurs. Methods and systems that facilitate identification of these and other cardiac episodes aid in delivering appropriate therapy to treat the disorders causing the episodes.